The execution of Clayton Lockett, 38, was stopped after 20 minutes, when one of his veins ruptured, preventing the drugs from taking full effect.

The execution of a fellow inmate, due two hours later, was postponed.

Both men had unsuccessfully challenged a state law that shields the identities of companies supplying the drugs.

The problems surrounding Lockett’s execution come amid a wider debate over the legality of the three-drug method and whether its use violates guarantees in the US constitution “against cruel and unusual punishment”.

Lockett writhed and shook uncontrollably after the drugs were administered, witnesses said.

“We believe that a vein was blown and the drugs weren’t working as they were designed to. The director ordered a halt to the execution,” Oklahoma Department of Corrections spokesman Jerry Massie said.

But Lockett’s lawyer, David Autry, questioned the remarks, insisting his client “had large arms and very prominent veins,” according to the Associated Press.

The prisoner was moving his arms and legs and straining his head, mumbling “as if he was trying to talk”, Courtney Francisco, a local journalist present at the execution, told the BBC.

Prison officials pulled a curtain across the view of witnesses when it became apparent that something had gone wrong.

“It was a horrible thing to witness. This was totally botched,” Mr Autry said.

Oklahoma Governor Mary Fallin said in a statement that she had ordered a full review of the state’s execution procedures.

Calls for investigation

Fellow inmate Charles Warner, 46, had been scheduled to be put to death in the same room two hours later in a rare double execution.

Warner’s lawyer, Madeline Cohen, who witnessed Lockett’s execution, said he had been “tortured to death” and called for an investigation.

“The state must disclose complete information about the drugs, including their purity, efficacy, source and the results of any testing,” she said.‏

Warner was convicted of the 1997 murder and rape of an 11-month-old girl.

He and Lockett had unsuccessfully challenged an Oklahoma state law that blocks officials from revealing – even in court – the identities of the companies supplying the drugs.

The state maintains the law is necessary to protect the suppliers from legal action and harassment.

Lockett and Warner argued they needed to know the names of the suppliers in order to ensure the quality of the drugs that would be used to kill them and to be certain that they had been obtained legally.

In March, a trial court ruled in their favour, but the state’s highest court reversed that decision last week, ruling that “the plaintiffs have no more right to the information they requested than if they were being executed in the electric chair”.

US states have encountered increasing problems in obtaining the drugs for lethal injections, amid an embargo by European pharmaceutical firms.

Some have turned to untried combinations of drugs or have sought to obtain the drugs custom-made from compounding pharmacies.

DETAILS

Prison officials in Oklahoma inserted lethal drugs into an inmate’s groin after failing to find a vein elsewhere during an execution that went wrong.

They took 51 minutes to find a suitable vein and did not realise the one they chose in his groin had collapsed.

Clayton Lockett succumbed to a heart attack 10 minutes after his execution was stopped, as he writhed and mumbled.

The corrections department disclosed new details of his last moments in a letter to the state governor.

It shows that Lockett fought with guards attempting to remove him from his cell on the morning of his execution.

He was subsequently shocked with a stun gun and taken to a prison infirmary, where a cut said to be self-inflicted was treated.

The intravenous line was placed in Lockett’s groin and covered with a sheet, according to prisons director Robert Patton’s letter to Oklahoma Governor Mary Fallin, which included a detailed timeline.

It became dislodged and was not discovered until 21 minutes after the execution began and all the lethal drugs had been administered.

“The drugs had either absorbed into tissue, leaked out or both,” Mr Patton wrote.

The doctor detected a faint heartbeat and Mr Patton halted the execution but it was too late to save Lockett’s life.

Another inmate initially scheduled for execution the same day as Lockett was granted a stay as the state continues to investigate the matter.

Charles Warner’s execution was initially rescheduled for 13 May, but Ms Fallin said on Thursday she was open to issuing a 60-day stay if need be.

“If it does require more time, then yes, I think they should take more time,” she told US media. “We need to get it right.”

Ms Fallin also ordered an “independent review” of the state’s execution protocols, saying the state’s department of public safety would lead the inquiry.

Her call for a review was echoed by Mr Patton in his letter.

Lockett was sentenced to death for shooting 19-year-old Stephanie Neiman and watching as two accomplices buried her alive in 1999.

Ms Neiman and a friend had interrupted the men as they robbed a home.

The problems surrounding Lockett’s execution come amid a wider debate over the legality of the three-drug method and whether its use violates guarantees in the US constitution “against cruel and unusual punishment”.

US states have encountered increasing problems in obtaining the drugs for lethal injections, amid an embargo by European pharmaceutical firms.

The triple-drug cocktail, first used in Texas in 1982, has become the standard execution method in the US.

It was presented as a more humane replacement for lethal gas and the electric chair, but critics of the three-drug protocol say it could cause unnecessary suffering.

Several US states that still have the death penalty have since switched to a single-drug method.